Modern mobile handsets or so called Smart-phones are not just capable of communicating over a wide range of radio frequencies and of supporting various wireless technologies. They also include a range of peripheral devices like camera, keyboard, larger display, flashlight etc. The provision to support such a large feature set in a limited size, constraints the designers of radio frequency (RF) front ends to make compromises in the design and placement of the antenna which deteriorates its performance. The surroundings of the antenna especially when it comes in contact with human body, adds to the degradation in its performance. The main reason for the degraded performance is the mismatch of impedance between the antenna and the radio transceiver which causes part of the transmitted power to be reflected back.
An antenna is connected through an RF front-end module (FEM) to the transceiver (TRx) to provide a bi-directional wireless RF link. Information is passed between the user and base-band controller via various user interfaces, like key-pads, microphone, loudspeaker and display. The baseband controller processes received data as well as data that needs to be transmitted and maintains synchronized connection to the cellular network. More specifically, the baseband controller modulates data packets for transmission and demodulates IF signals when received. The RF front-end module connects the antenna to selected transmitter (Tx) and receiver (Rx) signal paths that are frequency-band selective in order to minimize spurious emission and reception. The complexity of these front-end modules increases steadily because the number of mobile phone frequency-bands and communication standards keeps on getting larger leading to higher RF front-end loss and thus increased power consumption and poor sensitivity However, RF front-end performance may suffer from changes in the operating environment that are often unpredictable. Thus, by tuning a set of parameters, the analog front-end may support a given communication mode.
Tunable front end topologies been proposed and discussed for future generations of mobile terminals. However, the tuning of reactive band stop filter structures in the RF front-end has not been needed until now. The use of many parallel RF chains with fixed filters and antennas supports the large and increasing number of bands and standards. Traditional architectures lead to the use of a large number of different power amplifiers (PAs), switches, filters and low noise amplifiers (LNAs). This makes it difficult to reduce the area used for the RF solution in order to achieve attractive form factors for the market. Future capabilities calls for support for even more bands leading to an even more complex RF front-end.
The antenna match may be adaptively tuned using various tuning methods and circuitry, as implied above. However, such circuitry and methods cannot be used to tune the stop band part of the front end circuitry because the return loss, by nature, is very high in the part of the spectrum where band stop elements are located.